[PATCH RFC 3/3] x86/mm: Simplify/correct l1_disallow_mask()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

l1_disallow_mask() yields L1_DISALLOW_MASK with PAGE_CACHE_ATTRS conditionally
permitted.  First, rewrite it as a plain function.

Next, correct some dubious behaviours.

The use of is_pv_domain() is tautological; l1_disallow_mask() is only used in
the PV pagetable code, and it return true for system domains as well.

The use of has_arch_pdevs() is wonky; by making the use of cache attributes
dependent on the instantanious property of whether any devices is assigned,
means that a guest could have created a legal PTE which will fail validation
at a later point when the device has been removed.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Jan Beulich <JBeulich@xxxxxxxx>
CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>

RFC.  I've not tested this, and I doubt it will work to start with owing to
the removal of the dom_io special case which IIRC dom0 uses to map arbitrary
MMIO.

Furthermore, the rangeset_is_empty() calls have the same problem that
has_arch_pdevs() has; they're not invariants on the domain.  Also, VMs
wanting/needing encrypted memory need fully working cacheability irrespective
of device assignment.

I expect the way we actually want to fix this is to have a CDF flag for
allowing reduced cahcebility, and for this expression to simplify to just:

    if ( d->options & XEN_DOMCTL_CDF_any_cacheability )
        disallow &= ~PAGE_CACHE_ATTRS;

which is simpler still.

For the current form, bloat-o-meter reports:

  add/remove: 1/0 grow/shrink: 1/2 up/down: 75/-280 (-205)
  Function                                     old     new   delta
  l1_disallow_mask                               -      74     +74
  mod_l1_entry.cold                             55      56      +1
  get_page_from_l1e                           1271    1167    -104
  mod_l1_entry                                1860    1684    -176

which is an even bigger improvement than simply not duplicating the logic.
---
 xen/arch/x86/mm.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/xen/arch/x86/mm.c b/xen/arch/x86/mm.c
index 95795567f2a5..31937319c057 100644
--- a/xen/arch/x86/mm.c
+++ b/xen/arch/x86/mm.c
@@ -162,13 +162,17 @@ static uint32_t __ro_after_init base_disallow_mask;
 
 #define L4_DISALLOW_MASK (base_disallow_mask)
 
-#define l1_disallow_mask(d)                                     \
-    (((d) != dom_io) &&                                         \
-     (rangeset_is_empty((d)->iomem_caps) &&                     \
-      rangeset_is_empty((d)->arch.ioport_caps) &&               \
-      !has_arch_pdevs(d) &&                                     \
-      is_pv_domain(d)) ?                                        \
-     L1_DISALLOW_MASK : (L1_DISALLOW_MASK & ~PAGE_CACHE_ATTRS))
+static uint32_t l1_disallow_mask(const struct domain *d)
+{
+    uint32_t disallow = L1_DISALLOW_MASK;
+
+    if ( (d->options & XEN_DOMCTL_CDF_iommu) ||
+         !rangeset_is_empty(d->iomem_caps) ||
+         !rangeset_is_empty(d->arch.ioport_caps) )
+        disallow &= ~PAGE_CACHE_ATTRS;
+
+    return disallow;
+}
 
 static s8 __read_mostly opt_mmio_relax;
 
-- 
2.39.2